Tsunami

What are tsunami?

Tsunami are long period waves, most often generated by displacement of the sea floor associated with earthquakes, volcanic eruptions or landslides.

Tsunami can cause devastation to coastal communities with potential for destruction of coastal infrastructure, property and loss of life from the powerful waves and their inundation of land.

Damage from tsunami of unknown source to Berghan Family Store at Ahipara circa 1916 (Image: Far North Regional Museum).

What are the risks from tsunami?

Risk from tsunami in New Zealand is significant due to our exposure to significant tsunami-generating sources from the Pacific ‘Ring of Fire’ and our increasing amount of coastal development.

The amount of damage from tsunami is estimated to be about twice of what is expected from an earthquake event with a similar return period, with deaths and injuries likely to be many time more.

Have tsunami impacted Northland in the past?

A number of moderate-to-large tsunami have impacted Northland's coast, both historically and prehistorically, as recorded in oral traditions, written accounts and geological evidence.

In last 150 years, four moderate-to-large tsunami events have impacted Northland’s east coast.

Pre-historically, one large event, or a series of large, closely-spaced events, occurred around 600 years ago, along with other events around 3000 and 6500 years ago.

There are also likely to be other past tsunami events that have not been recorded or detected from geological evidence. One of several historic accounts (from F.Keene's book ‘To The Northward’ (1977), of how Taipa, located in Doubtless Bay, was named is “...many years ago a great tidal wave washed right over the pa on the seashore, temporarily submerging it. After the excitement of this catastrophe had died down, the survivors called the place Taipa (tai, sea-water; pa, village)”.

What is at risk in Northland?

Moderate-to-large historical tsunami events that impacted Northland occurred when development on the coast was modest, in comparison to today.

Significant coastal development since the last moderate historic event means there is now increased exposure to risk from tsunami hazard.

Any coastal-sited development or infrastructure, or coastal property is potentially at risk from tsunami, including in Northland, which has:

over 30 coastal settlements;

infrastructure such as water supply, sewage, power, telecoms, road and rail;

Above: Rapidly developing coast. Both images are of the same area of Ahipara, north of Wairoa Stream, showing the extent of development from 1950 (left) to 2004 (right).

What are we doing about it?

The Northland Regional Council has been working with scientists, planners and stakeholders to better understand the risk from tsunami to Northland, to enable us to better prepare for and manage the risk from tsunami hazard.

What have we done so far?

Review of prehistoric and historic tsunami record

Marine cobbles located on the dune up to 30m above MSL at Tom Bowling Bay, Far North, deposited from prehistoric tsunami of origin.

Northland's tsunami record was assessed by reviewing the literature, geological data - including sediment coring for assessment of prehistoric tsunami deposits - and a field reconnaissance of the Northland coast.

REPORTS:

Pearce, S. (2006) Sedimentary record of palaeotsunami from two backbarrier wetlands, east coast Northland. Unpublished MSc thesis, University of Auckland, SGES, 115pp.

Numerical modeling of tsunami wave height at coast

Several tsunami event scenarios were modelled for the Northland coast. The outputs from this work included wave height at coast, onshore current speed and wave arrival times. Tsunami inundation modelling has also been completed for 38 coastal settlements, involving simulation of several tsunami event scenarios.

Model Study 1 gave a broad outline of the methodology, and included tsunami mapping for Ahipara and Taipa, in the Far North District.

Model Studies 2 and 3 include tsunami mapping for a further 30 settlements, primarily on the East coast of Northland, where tsunami risk is considered highest.

Model Study 4 included tsunami mapping for the Whangarei harbour, from the Whangarei Heads to the CBD. Six areas on the harbour have been studied in detail.

The methodology and scenarios assessed is consistent for model studies 1 to 4. For several tsunami event scenarios, the model outputs include:

landward extent and water depth of tsunami inundation over land;

maximum current speeds from inundation over land;

wave arrival times; and

animations of inundation for some areas.

Obtaining Accurate Land Surface Information

To be able to model the way tsunami waves inundate land it is necessary to have accurate information on the surface of the land. To do this we obtained high resolution LiDAR topographic data for approximately 30 coastal communities in Northland for land that was generally below 20 metres elevation. LiDAR stands for Light Detection and Ranging. It uses the same principle as RADAR, except that it uses a laser instead of radio waves.

This data, obtained through aerial survey, is used to create a land surface model which is used in the numerical model to represent that land surface.